1. Field of the Invention
The present invention relates to an amplifying circuit and a touch control sensing system, and particularly relates to an amplifying circuit and a touch control sensing system which can suppress noise.
2. Description of the Prior Art
A touch control sensing system is applied to various kinds of electronic apparatuses, for example, a mobile phone, a tablet pc or a notebook. Many sensing mechanisms can be applied to the touch control sensing system for sensing a touch operation, for example, resistive sensing, capacitive sensing, or optical sensing.
FIG. 1 is a conventional touch control sensing system 100. The touch control sensing system 100 comprises a touch panel 101 utilizing capacitive sensing, and an amplifying circuit 103. Many devices are not illustrated here for the convenience for explaining. In such system, the drive clock DS is inputted to the touch panel 101 and will be pulsed during touch scanning. A current (i.e. a sensing current signal SC) will be generated during positive edge or negative edge of the drive clock DS.
The amplifying circuit 103 comprises an amplifier 105, a low pass filter 107 (ex. an anti alias filter), and an analog to digital converter 109. The sensing current signal SC will be picked up by the amplifier 105 to generate a sensing voltage signal SV to the low pass filter 107. The output of the low pass filter 107 will feed into the analog to digital converter 109 to convert to digital signal for further process. If an object such as a finger touches on touch panel 101, a capacitor between the object and the touch panel 101 will be generated. In such situation, partial of the sensing current signal SC generated by the drive clock DS will be absorbed by the amplifying capacitor Cf. Therefore, the sensing voltage signal SV decreases and it is determined that the “touch” is sensed.
Normally the cell capacitance Cc in the touch panel 101 is around 3 pf, thus the output of the amplifier 105 equals to Vdrv*(3 p/Cf). Vdrv indicates a drive voltage of the drive clock DS. However, no matter what electronic apparatus is the touch control sensing system 100 applied to, noise like common mode ground may disturb the capacitive touch performance. For example, if high amplitude noise exists, the sensing voltage signal SV may increase and get saturated. In such case, the sensing voltage signal SV equals to Vdrv*(3 pF/Cf)+Vnoise*(Cnoise/Cf). Cnoise indicates undesired capacitance such as the capacitance generated by the circuit. In order to suppress the noise, the capacitance of the amplifying capacitor Cf must be set to higher value. However, the amplifying capacitor Cf with higher capacitance causes a lower gain for the amplifier 105. Also, the amplifying capacitor Cf with higher capacitance increases the chip size and reduces the SNR (Signal to Noise Ratio) since the signal is also suppressed if the amplifying capacitor Cf has high capacitance.
Some techniques are developed to acquire a better SNR. For example, 128 data are collected per clock cycle for the drive clock to decide if the “touch” occurs or not. However, such operation need 32 drive clocks and each clock needs to be sampled 4 times per clock cycle, to generate 128 samples. Such method not only needs high report rate but also consumes more power.